1. Field of the Invention
This invention relates to a solid electrolyte fuel cell. More particularly, the present invention relates to a solid electrolyte fuel cell equipped with a solid electrolyte device which has electrodes formed on both surfaces of an oxygen ion-conductive solid electrolyte substrate and in which an oxygen-containing gas is supplied to the electrode on the cathode side of the solid electrolyte device while methane gas, as fuel, is supplied to the electrode on the anode side.
2. Description of the Related Art
Solid electrolyte fuel cells are expected to provide a higher power generation efficiency than the power generation efficiency of thermal power generation or the like. Therefore, numerous studies regarding fuel cells have been done. As shown in FIG. 10, a solid electrolyte fuel cell uses a burned body of stabilized zirconia containing yttria (Y2O3)(hereinafter called merely the xe2x80x9cYSZ burned bodyxe2x80x9d) as an oxygen ion-conductive solid electrolyte substrate 100. (This stabilized zirconia will be hereinafter called xe2x80x9cYSZxe2x80x9d in some cases.) The solid electrolyte fuel cell further includes a solid electrolyte device 106 having electrodes 102 and 104a formed on both surfaces of the solid electrolyte substrate 100.
Of the electrodes 102 and 104a of this solid electrolyte device 106, the electrode 102 is made of lanthanum strontium manganese oxide [ (La0.85Sr0.15)0.90MnO3], and is used as the cathode. Oxygen or an oxygen-containing gas is supplied to this electrode 102. The other electrode 104a comprises a porous platinum layer, and is used as the anode. Methane gas, as fuel, is supplied to this electrode 104a. 
Oxygen (O2) supplied to the electrode 102 of the solid electrolyte device 106 shown in FIG. 10 is ionized to oxygen ions (O2xe2x88x92) at the boundary between the electrode 102 and the solid electrolyte substrate 100. The oxygen ions (O2xe2x88x92) are transferred by the solid electrolyte substrate 100 to the electrode 104a. The oxygen ions (O2xe2x88x92) so transferred to the electrode 104a react with methane (CH4) gas supplied to the electrode 104a, forming water (H2O), carbon dioxide (CO2), hydrogen (H2) and carbon monoxide (CO). Since the oxygen ions emit electrons during this reaction, a potential difference develops between the electrode 102 and the electrode 104a. When the electrodes 102 and 104a are electrically connected to an external circuit 108, the electrons of the electrode 104a flow through the external circuit 108 to the electrode 102 (indicated by an arrow). Electric power can thus be obtained from the solid electrolyte fuel cell.
Incidentally, the operating temperature of the solid electrolyte fuel cell shown in FIG. 10 is approximately 1,000xc2x0 C.
The solid electrolyte device 106 shown in FIG. 10 has durability against the high operating temperature, but has low power generation performance such as terminal current density and discharge current density. Therefore, further improvements of the cell performance are required.
A solid electrolyte device 110 shown in FIG. 11 has been used. The solid electrolyte device 110 substantially comprises a solid electrolyte substrate 112 made of YSZ, and the electrode 104b as the anode is formed at one of the ends of this solid electrolyte substrate 112. The electrode 104b is made of the mixture of YSZ, that forms the solid electrolyte substrate 112, and cermet particles 114, 144 comprising of nickel (Ni) and nickel oxide (NiO).
Incidentally, the electrode 102 as the cathode, that is formed at the other end of the solid electrolyte substrate 112, is made of lanthanum strontium manganese oxide in the same way as the electrode 102 of the solid electrolyte device 106 shown in FIG. 10.
The solid electrolyte fuel cell using the solid electrolyte device 110 (hereinafter called the xe2x80x9cNi-YSZ cermet solid electrode device 110xe2x80x9d in some cases) shown in FIG. 11 has an improved power generation performance, such as discharge current density and terminal current density, in comparison with the solid electrolyte fuel cell using the solid electrolyte device 106 shown in FIG. 10.
However, the operating temperature of the solid electrolyte fuel cell using the solid electrolyte device 110 shown in FIG. 11, at which power can be obtained in a stable way, is at least about 920xc2x0 C. Power cannot be obtained stably at a temperature lower than 920xc2x0 C. The phenomenon of a gradual drop of activity of the solid electrolyte substrate 110 occurs at an operating temperature higher than 920xc2x0 C. Therefore, an improvement in the heat-resistant property of the solid electrolyte substrate 110 is required.
When a dry methane gas, after the removal of moisture, is supplied as fuel to the electrode 104b as the anode, the reactivity between the methane gas and the oxygen ions drops with the result that the solid electrolyte fuel cell fails to exhibit its full performance. For this reason, moisture-containing wet methane gas is supplied to the electrode 104b at present so as to secure reactivity between the methane gas and the oxygen ions.
The reaction between the methane gas and the high-temperature vapor is the endothermic reaction. Therefore, the temperature on the side of the electrode 140b drops, and carbon that is formed with the drop of the reaction temperature adheres to the electrode 104b and promotes a drop in activity of the solid electrolyte device 110. In other words, stable power generation is difficult.
It is therefore an object of the present invention to provide a solid electrolyte fuel cell that has an improved power generation performance, such as discharge current density and terminal current density, and has the maximum heat-resistance of the solid electrolyte device.
As a result of studies to solve the problem described above, the inventors of this invention have found that a solid electrolyte fuel cell using the solid electrolyte device 200 shown in FIG. 12 has an improved power generation performance, such as discharge current density and terminal current density, in comparison with the solid electrolyte fuel cell shown in FIG. 10, and can stably generate power even at an operating temperature of less than 920xc2x0 C. Some of the present inventors proposed a solid electrolyte fuel cell using the solid electrolyte device 200 shown in FIG. 12 in xe2x80x9cProgress in Battery and Battery Materialsxe2x80x9d, Vol. 17, April (1998), p. 137-143.
In the solid electrolyte device 200 shown in FIG. 12, electrodes 102 and 104c are formed on both surfaces of a solid electrolyte substrate 100 comprising a YSZ burned body. The electrode 102 used as the cathode is formed of lanthanum strontium manganese oxide [(La0.85Sr0.15)0.90MnO3]. Metal oxide particles 202, 202 made of PdCoO2 are blended in a porous platinum layer that forms the electrode 104c used as the anode.
However, power generation performance, such as discharge current density and terminal current density, of the solid electrolyte device 200 shown in FIG. 12, is not yet sufficient.
Therefore, the inventors of the present invention have further studied solid electrolyte fuel cells to improve the power generation performance, such as discharge current density and terminal current density, and the thermal and chemical stability of the solid electrolyte device. As a result, the present inventors have found that the power generation performance of the electrolyte fuel cell, such as discharge current density and terminal current density, and the heat-resistant property of the solid electrolyte device, can be improved remarkably when the solid electrolyte fuel cell has the solid electrolyte fuel device formed by blending metal particles of CoNiO2 in the porous platinum layer forming the electrode on the anode side, or the solid electrolyte fuel device has an oxide layer, in which metal oxide particles of PdCoO2 are sintered, on the surface of the porous platinum layer forming the electrode on the anode side. The inventors have thus completed the present invention.
In a solid electrolyte fuel cell of the type which is equipped with a solid electrolyte device having electrodes formed on both surfaces of an oxygen ion-conductive solid electrolyte substrate, and in which oxygen or an oxygen-containing gas is supplied to the electrode on the cathode side of the solid electrolyte device while methane gas, as fuel, is supplied to the electrode on the anode side, the present invention provides a solid electrolyte fuel cell having the construction wherein metal oxide particles consisting of CoNiO2 or CoO are blended as an oxidation catalyst for methane gas in the porous platinum layer that forms substantially the electrode on the anode side of the solid electrolyte device.
In a solid electrolyte fuel cell of the type which is equipped with a solid electrolyte device having electrodes formed on both surfaces of an oxygen-containing solid electrolyte substrate, and in which oxygen or an oxygen-containing gas is supplied to the electrode on the cathode side of the solid electrolyte device while methane gas, as fuel, is supplied to the electrode on the anode side, the present invention provides a solid electrolyte fuel cell having the construction wherein an oxide layer in which metal oxide particles consisting of PdCoO2 are sintered as an oxidation catalyst for methane gas is formed on the surface of the porous platinum layer that forms the electrode on the anode side of the solid electrolyte device.
In these inventions, it is suitable to form the solid electrolyte substrate of a stabilized zirconia burned body containing yttria, and to form the electrode on the cathode side of lanthanum strontium manganese oxide [(La0.85Sr0.15)0.90MnO3].
In the solid electrolyte fuel cell according to the present invention, the metal oxide particles of CoNiO2 or CoO, as an oxidation catalyst for methane gas, blended in the porous platinum layer forming the electrode on the anode side of the solid electrolyte device, or the metal oxide particles of PdCoO2 as an oxidation catalyst for methane gas, sintered into the oxide layer on the surface of the porous platinum layer, can promote the oxidation reaction of methane. Therefore, the solid electrolyte fuel cell according to the present invention can generate power more stably and over a broader range of the operating temperature, at which power can be generated stably, than the solid electrolyte fuel cell equipped with the Ni-YSZ cermet solid electrolyte device 110 shown in FIG. 11.
A dry methane gas, that need not contain moisture, can be used as the methane gas supplied as the fuel.
Among the solid electrolyte fuel cells according to the present invention, the solid electrolyte fuel cell equipped with the solid electrolyte device that contains the metal oxide particles of CoNiO2 or CoO blended in the porous platinum layer forming the electrode on the anode side is superior in power generation performance, such as discharge current density and terminal current density, and in thermal and chemical stability, to the solid electrolyte fuel cell equipped with the solid electrolyte device that contains the metal oxide particles of PdCoO2 sintered into the oxide layer on the surface of the porous platinum layer forming the electrode on the anode side.